1. Field of the Invention
The present invention relates to an image reading apparatus having a document feeding apparatus for conveying a original document.
2. Description of the Related Art
Conventionally, as described in for example, U.S. Pat. No. 6,405,017, a document reading apparatus having a document pressing plate provided such that the pressing plate is capable of being opened or closed in order to press a document against a contact glass has been well known. In the document reading apparatus having such a structure, the pressing plate is requested that the weight of the document pressing plate for pressing the document should be small and that the pressing face should be flat so as to keep the document into a firm contact with the contact glass throughout the entire range. Thus, in order to prevent warp or distortion of the document pressing plate from being transmitting to the pressing sheet, an elastic members is disposed between the document pressing plate and the pressing sheet member for pressing the document against the contact glass and further, the arrangement of such an elastic member has been devised in many ways.
Additionally, another image reading apparatus capable of selecting flow scanning type in which a document image is scanned while the original document is being conveyed and fixed scanning type in which a document image is scanned while the document such as a book page is fixed has been known also. Such an image reading apparatus is adopted in a copying machine and the like.
In this image reading apparatus, a document feeding apparatus is provided such that the apparatus is rotatable with respect to its reading apparatus main body.
In the case of the flow scanning type, the original document is conveyed by the document feeding apparatus and the document image is scanned by scanning means provided on the reading apparatus main body. On the other hand, in the case of the fixed scanning type, the document is placed on the document base and that document is pressed against the document base by the pressing plate provided on the back face of the document feeding apparatus and then scanned.
FIG. 11 shows a perspective view of the pressing plate in an image reading apparatus based on conventional technology.
In the case of the fixed scanning type, to scan an image at a high precision, it is necessary to make the entire face of the document (book pages) into a firm contact with the platen glass which is a document base for fixed scanning of the document.
For that purpose, the document needs to be pressed sufficiently by the pressing plate such that the entire face thereof never floats from the platen glass.
Conventionally, as shown in FIG. 11, a sponge 202, which is an elastic member, is attached to the entire range of the back face of a pressing plate 201. Then, when the pressing plate 201 is closed against the platen glass which is a document base for fixed scanning of the document, a pressing force for pressing the document by the pressing plate 201 is generated by a restoration force generated by elastic deformation of the sponge 202.
As shown in FIG. 12, a document feeding apparatus 200 capable of selecting the flow scanning type and the fixed scanning type has abutting means 205 for restricting a pass gap between the document feeding apparatus 200 and the image reading apparatus 203 by abutting against the image reading apparatus 203 in order to secure a predetermined pass gap at a scanning position in the case of the flow scanning type on the front side (right in FIG. 12) and the deep side (left in FIG. 12) of the document feeding apparatus 200.
In such a conventional image reading apparatus, adjustment for generating a uniform pressing force on an entire back face of the pressing plate 201 is required to heighten a scanning accuracy in the case of the fixed scanning type.
If the abutting condition of the abutting means 205 to the image reading apparatus 203 is adjusted to an appropriate one with the document feeding apparatus 200 closed, the sponge 202 is uniformly deformed elastically by only a predetermined amount so that the restoration forces Fa to Fc generated at that time are equalized.
Further, because the document feeding apparatus 200 is provided with a driving device including a document feeding motor and the like, its weight is quite large so that a large amount of operation force is required to open/close the document feeding apparatus 200 Such a document feeding apparatus 200 has a mechanism for applying a force in an opening direction of the document feeding apparatus 200 to a hinge unit 204 in order to relax the operation force when the document feeding unit is opened/closed.
When the document feeding apparatus 200 is closed, closing forces FA to FC obtained by subtracting the biasing force acting in the opening direction of the hinge unit 204 from a force descending due to the weight of the document feeding apparatus 200 act.
If the closing force of the document feeding apparatus 200 is smaller than a predetermined value, the sponge 202 attached to the entire back face of the pressing plate 201 cannot be crushed completely. Thus, not only it takes a long time to adjust the height to abut the document feeding apparatus 200 to the image reading apparatus 203, but also there is another problem that the document feeding apparatus may not be settled uniformly.
To prevent this phenomenon, the biasing force for biasing the document feeding apparatus 200 in the opening direction by the hinge unit 204 is weakened so that the closing force of the document feeding apparatus is relatively increased to be sufficiently larger than the restoration of the sponge.
However, there is such a problem that because the biasing force in the opening direction of the hinge unit 204 is set to weaken, a user's operation force for opening the document feeding apparatus 200 is increased.
Further, there is a problem that because the closing force is large, an impact sound generated when the document feeding apparatus is closed is increased.
Further, there is a problem that because it takes a larger cost if the sponge area is larger, reduction of the cost cannot be achieved.
As described above, according to the conventional technology, the structure of the pressing plate of the document feeding apparatus intensifies the operation force for opening the document feeding apparatus, thereby possibly leading to worsening of the operability.